Ionizing radiation is known to produce defects in semiconductors. For example, radiation generates unwanted holes and electrons in gate oxides and other oxide dielectric layers. Throughout the dielectric, radiation generates electron-hole pairs. Some of these electron-hole pairs will recombine while others will not, yielding free electrons and holes. If an irradiated dielectric is a gate oxide, by applying a negative voltage to the gate electrode, the electrons will move toward the substrate and the holes will move toward the gate electrode. If a positive voltage is applied to the gate electrode, the reverse will occur, the electrons will move toward the gate electrode and the holes will move toward the substrate. This movement of and subsequent trapping of electrons and holes on intrinsic trapping sites causes a shift in the threshold voltage due to the radiation. Radiation ultimately induces a build up of positive charge within the dielectric due to large capture cross-sections of hole traps. Various methods have been employed to form radiation hard gate oxides to compensate for the build up of positive charges and to prevent such shifts in the threshold voltage from occurring when the integrated circuit or device is subjected to radiation.
One such method of forming radiation hard gate oxides is to form a nitrided oxide or oxidized nitrided oxide. While no definitive explanation appears to be able to explain all of the observed phenomena for this approach, two plausible explanations exist as to why the oxide becomes radiation hard as more fully described in "Role of Electron Traps in the Radiation Hardness of Thermally Nitrided Silicon Dioxide," by K. Ramesh et al, IEEE Electron Device Letters, Vol. 12, Dec. 1991. One explanation suggests that during nitridation, the oxide is structurally changed which results in fewer hole traps to capture the holes generated by radiation. Another explanation, as further described in "Rapid-Thermal Nitridation of SiO.sub.2 for Radiation-Hardened MOS Gate Dielectrics," by R. Sundaresan et al, IEEE Transactions on Nucclear Science, Vol. NS-30, page 4141, 1983, suggests that large electron traps are formed which capture the electrons generated by radiation and compensate for the trapped holes. Additional methods of forming radiation hard dielectrics include gate oxide formation in the presence of steam and gate oxide formation in ultra-dry high temperatures which excludes hydrogen. These method are known to either reduce hole traps or increase electron traps.
Ramesh et al demonstrated that electron trapping does play a significant role in radiation hardness of nitride oxides. This invention describes a new method of forming radiation hard gate oxides and dielectric layers by implanting silicon into the silicon dioxide to increase the number of electron traps.
it is therefore an object of this invention to provide a method of forming radiation hard dielectrics by implanting silicon into silicon dioxide regions.
it is a further object of this invention to provide such a method which utilizes conventional process flows.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.